So I'm sitting around depressed because the house is cold - I have this S33 problem - and my team just lost . After reading all the advice here I concluded I have an Air Pressure Switch fault .
On ebay there's a used one for £7 but I go and try to find my APS just to check .
I take the white metal cover off the boiler . I unclip the 3 clips holding the aluminium sheet with " Vaillant " stamped onto it and I gently pull that sheet of aluminium off . And the boiler fires up straight away !!!!!!
Don't ask me to explain . Maybe what I did was wrong . But my house is warming up very nicely .
Thank you men !!!!!
Terranss
January 2016